The present invention relates to a rubber composition and a pneumatic tire.
In recent years, from environmental issues such as global warming, social requirements for low fuel consumption efficiency have been enhanced, and development of low fuel efficient tire in which rolling resistance is lowered has been desired in response to low fuel consumption efficiency of an automobile.
However, in general, a rubber composition lowering rolling resistance by reducing stress and hysteresis loss at low tensile elongation (at low strain) of a tire such as at low speed running decreases stress and hysteresis loss also at high tensile elongation (at high strain) of a tire such as sudden brake, and dry grip performance can not be improved. As a result, it has been difficult to have both lowering of rolling resistance and improvement in dry grip performance.
As a method of reducing rolling resistance of a tire, an technique of substituting carbon black with silica is known. However, in the rubber composition compounding silica, it is known that dry grip performance is lowered, and when repeated running, grip performance is further more lowered due to decreasing rigidity of a rubber. Also, since a silanol group which is a surface functional group of silica forms a hydrogen bond, silica tends to coagulate each other, and there is a problem such that a Mooney viscosity of a rubber composition becomes high and processability such as extrusion is inferior since dispersion of silica particles in a rubber becomes insufficient.
In order to solve theses problems, silica has been employed together with various coupling agents, a dispersing agent, a surface modifier and the like. For example, a silane coupling agent is considered to prevent silica particles from coagulating each other due to bonding with silanol groups on the surface of silica and to improve processability of the rubber composition. However, no rubber composition in which dry grip performance can be improved besides rolling resistance is lowered has been put into practical use yet.
JP-A-8-337687 discloses a rubber composition lowering rolling resistance by compounding particulate silica and a silane coupling agent, however, the rubber composition is insufficient in the improvement effects of wet grip performance and dry grip performance, and thus, it still has a scope to be improved.